1. Field of the Invention
This invention relates to a process for removing volatile contaminants from the vadose zone, also known as the zone of aeration or the unsaturated zone, that is, the zone between the earth's surface and the underground water table.
In the following description, the term "volatile liquid contaminant" shall mean the liquid phase of the contaminant and the vapor phase in equilibrium therewith in the subsurface media.
Contamination of the subsurface ground with potentially hazardous materials is a common problem facing industry, the government and the general public. Often, as a result of spills, leakage from storage facilities or surface discharges, contaminants enter subsurface soil and/or rock formations. These contaminants eventually percolate into the groundwater, thereby posing a health threat to drinking water supplies. Various methods for withdrawing and treating contaminated groundwater are well known and widely practiced. However, methods for decontaminating subsurface soil and/or rock located in the vadose zone, above the water table, are limited. Generally, the contaminated soil in the vadose zone is excavated and removed. Alternatively, the contaminated soil can be left in place so that the contaminants can be leached out by percolating water and then the contaminated percolating water can be removed and recovered after it has reached the water table. The leaching of contaminants from subsurface media is extremely slow for many common contaminants because of their low solubility in water and their absorption by soil and rock formations. As a result, remedial measures, directed at cleaning up and protecting the groundwater near a contamination site, must be performed for many years or even decades. In addition, the prior methods are ineffective for removing, from the vadose zone, contaminants having a low solubility in water and/or a high affinity for soil, which class of contaminants includes most of the hazardous pollutants that affect groundwater. The present invention involves removing volatile contaminants directly from the subsurface media in the vadose zone above the water table.
2. Description of the Prior Art
Numerous techniques exist for removing gas that is formed naturally in landfills and other constructed media. Circulation systems for leaching the contaminants from contaminated media above the water table have been designed, but they invariably require that the leached contaminants enter the water table or aquifer and be recovered by means of a water removal well and pump that draws water from the saturated zone or aquifer (see, for example, Forte et al, U.S. Pat. No. 4,167,973). In addition, it has been proposed to remove contaminant vapors from contaminated underground areas by means of a plurality of elongated perforated collection elements installed in a large excavation (see Knopik, U.S. Pat. No. 4,183,407). However, this system is limited to removal of contaminants from depths for which excavation is practical, usually less than about 25 feet deep.
The present invention provides a method for removing volatile contaminants from subsurface media, directly from any depth, within the vadose zone, without effecting hazardous and expensive excavation of a large amount of contaminated soil or rock. The equipment used to perform the present invention can be conventional equipment, most of which can be installed on the surface of the ground. Thus, the equipment need not be specially manufactured or installed within the contaminated area beneath the ground surface. The equipment can be installed with minimal disruption of the subsurface media. This is especially important in industrial environments in which excavation may be detrimental to industrial operations or the integrity of industrial structures in the area of contamination.